1. Field of the Invention
This invention relates to the removal of hydrogen sulfide from gas streams by contacting the gas with an aqueous washing solution, and more particularly to an improved method for the removal of hydrogen sulfide from gas streams in which the undesirable formation of water-soluble sulfur-containing salts and the undesirable chemical consumption of the constituents of the washing solution are substantially reduced.
2. Description of the Prior Art
With the increasing concern over the pollution of the atmosphere, great demands have been made on industry to produce pollution-free products in a pollution-free manner. One area of particular concern has been the release of sulfur and its compounds into the atmosphere as a result of the refining of petroleum, the sweetening of sour natural gas, the processing of ores, the destructive distillation of coal and/or oil shale, the gasification or liquefaction of coal, the use of geothermal fluids to generate electricity, or other processes which produce hydrogen sulfide-containing gases.
Various processes have been employed to remove hydrogen sulfide from gas streams. The Stretford process and the Takahax process employ aqueous alkaline washing solutions to absorb the hydrogen sulfide from the gas stream and to convert the absorbed hydrogen sulfide to elemental sulfur. Various methods have been used to separate the produced sulfur from the washing solution, including (1) direct-autoclaving in which an aqueous slurry comprised of sulfur and the washing solution is fed directly to an autoclave, and (2) indirect-autoclaving in which the washing solution is initially separated from the sulfur product by filtration or the like and the resulting sulfur cake is reslurried with water for introduction into an autoclave. In the autoclave, the aqueous sulfur slurries are heated to melt the sulfur and form dense liquid sulfur which separates from the aqueous carrier liquid by gravity.
While these processes are successful in substantially reducing the hydrogen sulfide content of the gas streams being treated, several problems exist which tend to increase their operating expenses. One such problem involves the chemical consumption of the washing solution constituents. The cost of chemicals required to replenish the supply of these consumable constituents adds significantly to the cost of the process. Another of the problems involves the undesirable conversion of the absorbed hydrogen sulfide and/or the product sulfur to water-soluble sulfur-containing salts, such as sulfates, thiosulfates and polythionates, which accumulate in the washing solution. These accumulated salts tend to (1) increase the corrosivity of the washing solution, (2) cause precipitation of the desired constituents of the washing solution and (3) increase the rate of chemical consumption of the alkaline constituents of the washing solution.
In commercial plants employing these processes, attempts have been made to mitigate these problems either by periodic replacement of the washing solution or by intermittently or continuously bleeding a slipstream of the contaminated washing solution while adding make-up chemicals to replenish the circulating solution. Both procedures are unsatisfactory due to the expense of the make-up chemicals and the difficulty involved in disposal of the contaminated solution.
Recent modifications of the washing solutions employed to treat hydrogen sulfide-containing gas streams have resulted in a substantial reduction or elimination of the aforementioned problems when the product sulfur is separated from the washing solution by means other than autoclaving with recycle of the aqueous autoclave effluent. When an autoclave is used, however, the aqueous slurry fed into the autoclave usually contains some constituents of the washing solution, such as vanadium, a quinone and/or alkaline constituents, which tend to catalyze the reverse Claus reaction whereby elemental sulfur is converted to water-soluble sulfur-containing salts, such as thiosulfates and polythionates. Colloidal vanadium may also be formed in the autoclave. Colloidal vanadium and sulfur-containing salts in the recycled aqueous autoclave effluent tend to reduce the efficiency of the washing solution.
While these problems associated with the use of an autoclave often render it impractical to use direct-autoclaving, and in some cases even indirect-autoclaving, the use of an autoclave is attractive from an operational standpoint because autoclaves generally require less operator attention and maintenance than a filter, a centrifuge or the like which would otherwise be employed. Thus a need exists for a hydrogen sulfide removal and conversion method employing an autoclave while avoiding the aforementioned problems.
Accordingly, a primary object of this invention is to provide a method for removing hydrogen sulfide from gas streams wherein a liquid sulfur product is recovered using an autoclave while avoiding the problems heretofore associated with the use of an autoclave.
Another object of this invention is to provide a hydrogen sulfide removal method in which the hydrogen sulfide content of the gas being treated is reduced to an acceptably low level at a reasonable cost.
Still another object of this invention is to provide a method for removing hydrogen sulfide from gas streams in which the absorbed hydrogen sulfide is converted substantially exclusively to sulfur and wherein the undesirable conversion of hydrogen sulfide and/or the product sulfur to water-soluble sulfur-containing salts is substantially reduced or eliminated.
Yet another object of this invention is to provide a method in which hydrogen sulfide is absorbed from a gas stream by a regenerable washing solution and in which the chemical consumption of the washing solution constituents is substantially reduced or eliminated.
A further object of this invention is to provide a hydrogen sulfide removal and conversion method employing an autoclave to recover a liquid sulfur product, which method avoids the undesirable formation of chemical species which are detrimental to the efficiency of the washing solution.
Still further objects, advantages and features of the invention will become apparent to those skilled in the art from the following description when taken in conjunction with the accompanying drawing.